Electrochemical devices or systems of the type referred to herein include one or more of the metal-halogen battery systems, such as a zinc-chloride battery system. These metal-halogen battery systems generally are comprised of three basic components, namely an electrode stack section, an electrolyte circulation subsystem, and a store subsystem. The electrode stack section typically includes a plurality of cells connected together electrically in various series and parallel combinations to achieve a desired operating voltage and current at the battery terminals over a charge/discharge battery cycle. Each cell is comprised of a positive and negative electrode which are both in contact with an aqueous metalhalide electrolyte. The electrolyte circulation subsystem operates to circulate the metalhalide electrolyte from a reservoir through each of the cells in the electrode stack in order to replenish the metal and halogen electrolyte ionic components as they are oxidized or reduced in the cells during the battery cycle. In a closed, self-contained metal-halogen battery system, the storage subsystem is used to contain the halogen gas or liquid which is liberated from the cells during the charging of the battery system for subsequent return to the cells during the discharging of the battery syetem. In the zinc-chloride battery system, chlorine gas is liberated from the positive electrodes of the cells and stored in the form of chlorine hydrate. Chlorine hydrate is a solid which is formed by the store subsystem in a process analogous to the process of freezing water where chlorine is included in the ice crystal.
With reference to the general operation of a zinc-chloride battery system, an electrolyte pump operates to circulate the aqueous zinc-chloride electrolyte from a reservoir to each of the positive or "chlorine" electrodes in the electrode stack. These chlorine electrodes are typically made of porous graphite, and the electrolyte passes through the pores of the chlorine electrodes into a space between the chlorine electrodes and the opposing negative or "zinc" electrodes. The electrolyte then flows up between the opposing electrodes or otherwise out of the cells in the electrode stack and back to the electrolyte reservoir or sump.
During the charging of the zinc-chloride battery system, zinc metal is deposited on the zinc electrode substrates and chlorine gas is liberated or generated at the chlorine electrode. The chlorine gas is collected in a suitable conduit, and then mixed with a chilled liquid to form chlorine hydrate. A gas pump is typically employed to draw the chlorine gas from the electrode stack and mix it with the chilled liquid, (i.e., generally either zinc-chloride electrolyte or water). The chlorine hydrate is then deposited in a store container until the battery system is to be discharged.
During the discharging of the zinc-chloride battery system, the chlorine hydrate is decomposed by permitting the store temperature to increase, such as by circulating a warm liquid through the store container. The chlorine gas thereby recovered is returned to the electrode stack via the electrolyte circulation subsystem, where it is reduced at the chlorine electrodes. Simultaneously, the zinc metal is dissolved off of the zinc electrode substrates, and power is available at the battery terminals.
Over the course of the zinc-chloride battery charge/discharge cycle, the concentration of the electrolyte varies as a result of the electrochemical reactions occurring at the electrodes in the cells of the electrode stack. At the beginning of charge, the concentration of zinc-chloride in the aqueous electrolyte may typically be 2.0 molar. As the charging portion of the cycle progresses, the electrolyte concentration will gradually decrease with the depletion of zinc and chloride ions from the electrolyte. When the battery system is fully charged, the electrolyte concentration will typically be reduced to 0.5 molar. Then, as the battery system is discharged, the electrolyte concentration will gradually swing upwardly and return to the original 2.0 molar concentration when the battery system is completely or fully discharged.
Further discussion of the structure and operation of zinc-chloride battery systems may be found in the following commonly assigned patents: Symons U.S. Pat. No. 3,713,888 entitled "Process For Electrical Energy Using Solid Halogen Hydrates"; Symons U.S. Pat. No. 3,809,578 entitled "Process For Forming And Storing Halogen Hydrate In A Battery"; Carr et al U.S. Pat. No. 3,909,298 entitled "Batteries Comprising Vented Electrodes And Method of Using Same"; Carr U.S. Pat. No. 4,100,332 entitled "Comb Type Bipolar Electrode Elements And Battery Stack Thereof". Such systems are also described in published reports prepared by the assignee herein, such as "Development of the Zinc-Chloride Battery for Utility Applications", Interim Report EM-1417, May 1980, and "Development of the Zinc-Chloride Battery for Utility Applications", Interim Report EM-1051, April 1979, both prepared for the Electric Power Research Institute, Palo Alto, Calif. The specific teachings of the aforementioned cited references are incorporated herein by reference.
As discussed in detail in the foregoing reports, conductivity-improving salts such as potassium chloride and sodium chloride are typically added to the electrolyte and zinc-chloride battery systems to improve overall energy efficiency. In a preferred electrolyte having a two molar concentration of zinc-chloride (when the battery system is fully discharged), it is desirable to use increasingly high concentrations of such supporting salts, such as a four molar concentration of potassium chloride and a one molar concentration of sodium chloride, to further improve energy efficiency. Experiments with existing zinc-chloride battery systems, such as integrated load-leveling modules, revealed that such highly salted (i.e., four to five molar supporting salts) electrolytes caused the hydrate decomposition heat exchanger to seriously clog and/or eventually plug. This stopped the flow of electrolyte through the heat exhanger, which in turn halted the decomposition of chlorine hydrate such that the battery system would no longer produce electricity. The decomposition heat exchangers are typically coiled titanium tubes and are used to transfer heat from the warm electrolyte in the reservoir or sump of the electrode stack section of the battery system to the chlorine hydrate in order to decompose the hydrate and thereby recover chlorine gas. In existing zinc-chloride battery systems, the decomposition heat exchanger is found or located level with or below the sump of the electrode stack section typically in the bottom of the store section of the battery system.
Analysis of the plugged heat exchangers showed that the supporting salts, particularly potassium chloride, had precipitated out of the highly salted electrolyte solution, and had tended to collect or agglomerate. Specifically, when the flow of electrolyte through the decomposition heat exchanger stopped, most of the electrolyte flowing therethrough remained in the heat exchanger. The supporting salts then precipitated or crystallized out of the stagnant electrolyte within the heat exchanger, apparently a result of a super-saturated condition which occurred when the temperature of the electrolyte approached the internal temperature of the store section (which is preferably kept at about ten degrees C.). In normal operation of the battery system during discharge mode, such precipitation does not occur when electrolyte is circulating through the decomposition heat exchanger. Apparently the warm electrolyte from the sump of the electrode section, which is typically at thirty to forty degrees C., is not cooled sufficiently during its passage through the decomposition heat exchanger to cause precipitation of the supporting salts in flowing electrolyte.
Accordingly, it is a principal objective of the present invention to provide an improved battery system design which is capable of long term operation using highly salted electrolytes without clogging or plugging of the decomposition heat exchanger.
Accordingly, it is a principal objective of the present invention to provide an improved battery system design which is capable of long term operation using highly salted electrolytes without clogging or plugging of the decomposition heat exchanger.
It is a more specific objective of the present invention to provide an improved decomposition heat exchanger arrangement which is capable of draining itself of electrolyte when electrolyte is no longer flowing therethrough.
It is a further objective of the present invention to provide the foregoing self-draining feature for the decomposition heat exchanger without resorting to any electrical controls.
It is another objective of the present invention to provide an improved battery system capable of using highly salted electrolyte without clogging or plugging the heat exchanger without appreciably reducing overall system efficiency, or adding appreciably to the system cost.
Yet another objective of the present invention is to provide a method to eliminate or minimize precipitation of conductivity-improving salt in the decomposition heat exchanger.
To achieve the foregoing objectives, the present invention provides an improved battery system which features (in addition to an electrode stack section or means, a store section or means, and electrolyte circulation and hydrate formation subsystems or means) a decomposition heat exchanger positioned higher than the electrolyte collected in the sump of the electrode stack section to allow the electrolyte to drain out of the heat exchanger when the electrolyte is not being circulated therethrough. The improved battery system may also include a vent connected to the gas space of the stack section that is located between the outlet of the electrolyte pump and inlet of the heat exchanger to improve drainage of the heat exchanger by allowing gas to enter heat exchanger to replace the electrolyte which drains therefrom.
Additional advantages and features for the present invention will become apparent from a reading of the detailed description of the preferred embodiments which make reference to the following set of drawings in which :